Paint is typically applied by spraying a mixture of paint solids and solvents onto a target such as an automobile. During this operation, some of the paint solids are not transferred to the target. Paint not reaching the target is referred to as overspray.
Because of increasing environmental concerns and regulations regarding the disposal of industrial waste products, several different methods have been developed aimed at reducing the volume of overspray generated in the course of painting an automobile. Additionally, processes have been developed which focus on recovery of the paint solids from both the overspray and from the cleaning of the spray equipment and the spray booth so that both the paint solids and the solvent can be reutilized in the coating process.
A common process for the recovery of paint solids from overspray utilizes a down-draft paint booth having an associated venturi scrubber and an eliminator section. Paint overspray continuously contacts a liquid circulating in the spray booth. Traditionally, the liquid is water. Then, high velocity filtered and conditioned air is supplied to the booth which exits the spray booth through a venturi scrubber. At the venturi scrubber, the high velocity air contacts the water for the purpose of scrubbing, i.e., removing the paint particulates. Then, the high velocity air enters an eliminator section which is typically disposed down stream of the scrubber section. The eliminator section includes a series of baffles which cause the disengagement of water droplets and paint solids that have become entrained in the air stream.
Paint overspray is typically recovered from water through chemical detackification. In the detackification process, chemicals such as organic polymers, clays, or silicas are added to the booth water, at a controlled pH, in order to detackify the paint, thus rendering the solids non-sticky. The chemical detackification process renders the overspray non-sticky or "detackified." By and large, the chemicals used in this method enhance the flotation of the paint solids through their coagulation and flocculation. Chemicals such as clays cause the paint solids to sink from the booth water where they could be collected as sludge. The sludge is allowed to settle in a retention tank and is then taken to a landfill for disposal and the solvent is removed.
Another method for the control of overspray and the recovery of solvents and paint solids therefrom replaces the water and chemical detackifying chemicals with an emulsion. The use of emulsion technology has eliminated the necessity for sludge dewatering as spent emulsion is taken off-site for treatment and disposal.
In a typical emulsion system, the circulating booth liquid is an emulsion containing approximately thirty percent by weight naphthenic oil and approximately seventy percent by weight water. Paint overspray solids are continuously contacted with the emulsion. The concentration of suspended solids in the emulsion can nominally be maintained in the range of five to ten percent by weight of solids. In order to maintain this concentration, a portion of the emulsion laden with paint solids, "spent emulsion," is removed from the system and is taken to an off-site facility where it is processed. At the off-site facility, the emulsion is chemically broken or cracked by the addition of sulfuric acid to form a sludge phase, an oil or solvent phase, and a water phase which are subsequently fractionated for recovery and/or disposal. This is an effective method for the recovery of paint solids from overspray; however, the use of off-site recovery of the spent emulsion involves increased costs due to the on-site storage and the transportation of the spent emulsion to an off-site processing plant.
Accordingly, it would be desirable and advantageous to have a process for recovering spent emulsion on-site, at the point where it is generated, in order to eliminate the costs associated with the prior art methods of overspray management and fluid recovery.